Utilizing triethylenetetramine-functionalized MIP-206 for highly efficient removal of Pb(II) from wastewater

The global concern over heavy metal pollution necessitates urgent measures to safeguard human health and the environment. This study focuses on employing triethylenetetramine (TETA)-functionalized MIP-206-OH (TMIP-206) as an effective adsorbent for removing Pb(II) from wastewater. TMIP-206 was synthesized via a hydrothermal method followed by functionalization with TETA. Kinetic studies demonstrate that lead removal on TMIP-206 conforms to the pseudo-second-order model, indicating an efficient removal process. Experimental results reveal that TMIP-206 aligns with the Langmuir isotherm, exhibiting a maximum removal capacity of 267.15 mg/g for lead ions. The sorption efficiency of TMIP-206 for Pb ions remains stable across six cycles, with a reduction of less than 15%. Optimal adsorption performance is observed at a pH of 6. These findings underscore the potential of TMIP-206 as an alternative for adsorbing Pb(II) from aqueous environments, addressing the global challenge of heavy metal pollution. Future research should explore the scalability and long-term stability of TMIP-206-based adsorbents to enhance their practical applicability in diverse environmental contexts and contribute to broader strategies for mitigating heavy metal contamination.

demonstrated to be insufficient and suffer from a rather slow process 35 .Also, chemical precipitation lacks the required selectivity for metal removal and requires large consumption of chemicals [36][37][38] .Numerous materials have found extensive application in the adsorptive elimination of heavy metals from aqueous sources, including altered activated carbon, carbon nanotubes, zeolites, and metal-organic frameworks (MOFs) [39][40][41][42] .In recent times, there has been significant attention given to exploring the potential of MOFs as adsorbents for pollutants 42   .MOFs are three-dimensional materials constructed from ligand linkers and metal ions as central components 43   .Because of their distinctive features, including customizable surface functionalization MOFs, elevated porosity, and extensive surface area serve as exceptional porous frameworks for applications such as gas storage, catalysis, photocatalysis, drug delivery, and adsorption-driven separation processes 42 .Functional groups like nitro, amine, thiol, and sulfur, incorporating nitrogen (N) and sulfur (S) atoms, exhibit notably efficient interactions with heavy metals, particularly lead ions due to functioning as Lewis bases 44 .In this context, numerous researchers have recently begun evaluating the side functionalization of MOFs and their effectiveness as adsorbents for heavy metals in aquatic environments.Zheng and team designed nanofibrous membranes composed of polyethersulfone and modified with ionic liquids for the purpose of heavy metal removal.Their study highlighted the potential of these innovative membranes with coordination of nitrogen atoms in addressing heavy metal contamination 45 .In a separate investigation, Mohammadi and co-authors conducted the synthesis of copper-based metal-organic frameworks (Cu-MOFs) with amine modifications, aiming to removal of lead ions from effluent samples 44 .The porous structure of the MOF, coupled with amine functionalization, acted as an optimal platform for the effective removal of lead ions 44 .Ke Wang and colleagues synthesized Zr-MOFs functionalized with -NH 2 groups and employed them for the first time in adsorbing both Pb and Cd ions 46 .It is evident that functional groups significantly contribute to augmenting the adsorption capacity in the majority of well-known MOFs used as heavy metal adsorbents.Despite these improvements, the pore network morphology of these MOFs suffers from a narrow pore opening and the positioning of functional groups.The ideal pore framework for heavy metal removal can be homogenous pores with open windows containing active sites facing the pore interior, shaping a cage-like structure with clips facing the cage center to keep metal ions inside.Recently, Wang et al., developed a MOF based on Zr clusters and isophthalic acid (IPA) named MIP-206.MIP-206 exhibited mesopores with open entrance as well as remarkable chemical and thermal stability.However, there was a drawback in synthesizing MIP-206 with amine functionalized IPA leading to non-uniform structural phases.Also, its structural stability in water media has yet to be explored.Having considered the advantages offered by MIP-206, a research avenue opens to explore various applications of this material and its behavior in different environments.
The novelty of this research lies in the functionalization of MIP-206-OH with triethylenetetramine, which has not been previously explored for Pb(II) adsorption.This novel approach enhances the adsorption capacity and reusability of the material, offering a promising solution for mitigating heavy metal pollution in aqueous environments.In this study, an initial implementation of triethylenetetramine on MIP-206 (TMIP-206) was performed to incorporate amine moieties into its pore network.TMIP-206 was applied for the adsorptive elimination of lead ions from wastewater, as a representative of heavy metal ions.The resulting product can capitalize on the benefits derived from the amine functional groups present in triethylenetetramine and the porous crystalline framework of MIP-206.Potential experimental factors affecting adsorption were thoroughly examined.The products were characterized using BET, FTIR, XRD, TGA, and SEM.The water stability of MIP-206 and TMIP-206 was analyzed by monitoring its crystallinity using XRD.Furthermore, laboratory data were analyzed with the widely recognized Freundlich and Langmuir isotherms to comprehend the nature of adsorption.

Methodology for adsorbent preparation
In the experimental procedure, 5-OH-IPA weighing 1.44 g and corresponding to 8.0 mmol was meticulously placed into a 46 mL Teflon reactor.Following this, 10 mL of formic acid (FA) was introduced, initiating a stirring process for a period of 5 min at ambient temperature until the formation of a homogeneous suspension was achieved.Subsequently, ZrOCl 2 •8H 2 O weighing 2.86 g and equating to 12 mmol was introduced to the suspension, and the mixture underwent an additional 10 min of stirring at ambient temperature to ensure the uniform dispersion of reactants.
The ensuing reaction mixture was then hermetically sealed within an autoclave and subjected to a gradual temperature increase, reaching 200°C over a period of 2 h, followed by a sustained maintenance at 200°C for a duration of 20 h.Upon cooling to room temperature, the anticipated product, denoted as MIP-206-OH, was obtained in the form of a filtrate weighing 4 g.This filtrate underwent a purification process involving washing with acetone and subsequent air-drying.
The activation process of MIP-206-OH commenced at 120°C through a vacuum heating procedure extending over 12 h.In a separate phase of the experiment, a precisely calculated quantity of TETA was dissolved in 20 mL of CHCl 3 .Subsequently, 200 mg of the activated MIP-206-OH was introduced into the solution, initiating a stirring period lasting 0.5 h.Following this interaction, the solution underwent a meticulous filtration process

Methodology for lead adsorption isotherms
The examination of isotherms was performed utilizing data obtained from lead uptake experiments involving TMIP-206.The data on lead uptake underwent analysis using the Freundlich, Langmuir, and Temkin equations, and the parameters for fitting were computed.The equations employed for the isothermal investigation are outlined as follows: In this context, q e (mg/g) signifies the lead uptake at equilibrium, q m (mg/g) signifies the theoretical optimum value for lead uptake, C e (mg/L) represents the lead uptake at equilibrium, and K l (L/mg) stands for the Langmuir fitting parameter.Additionally, K f (mg/g) and n represent the Freundlich parameters, where A T (L/g), b T , T (K), and R (8.314 J/mol/K) represent the Temkin isotherm equilibrium binding constant, Temkin isotherm constant, temperature, and universal gas constant, respectively.
The impact of reaction duration was examined through lead uptake experiments utilizing 200 mg/L of TMIP-206, a lead concentration ranging from 5 to 100 mg/L, and a sample volume of 100 mL.

Methodology for lead adsorption kinetics
The influence of reaction duration was assessed by conducting lead uptake experiments employing 200 mg/L of TMIP-206, a lead concentration of 100 mg/L, a sample volume of 100 mL, pH of 6, temperature of 25 °C, and reaction times spanning from 1 to 180 min.Following the separation of the adsorbent, the residual lead content was determined.Subsequently, the lead uptake performance of TMIP-206 was computed.The acquired data underwent analysis using pseudo-first order, pseudo-second order, and Elovich kinetic models.The corresponding fitting parameters were computed.The respective formulations for the kinetic models are detailed in Eqs. ( 6), ( 7) and ( 8).
( 1) www.nature.com/scientificreports/ In this context, q t (mg/g) signifies the lead uptake performance at time t, q m (mg/g) represents the theoretical lead uptake capacity, while K 1 (1/min) and K 2 (g/mg.min)are the respective fitting parameters for the kinetic equations.In the Elovich kinetic model, α (mg/g.min)denotes the initial adsorption rate, while β denotes the desorption constant.

Methodology for pH study on lead adsorption
The impact of varying pH levels in water samples was explored through lead uptake experiments employing TMIP-206.These experiments included an application of 200 mg/L of TMIP-206, a pH range spanning from 2 to 8 and a Pb ion concentration of 100 mg/L.Adjustments to pH were achieved by 0.04 M HCl or NaOH.The residual lead content in each sample was quantified, and the lead uptake capacity was calculated using Eq. ( 1).

Methodology for recovery study
For the regeneration of the adsorbent, it underwent a rinsing process using distilled water, followed by elution with a 1M NaOH solution to eliminate the adsorbed lead ions.TMIP-206 underwent repeated washing and elution cycles until the eluent no longer exhibited the presence of lead ions.Subsequently, the regenerated adsorbent was subjected to a drying procedure, rendering it ready for subsequent reuse.

Methodology for characterization
Nitrogen gas adsorption isotherms were acquired with a Micromeritics TriStar II Plus gas adsorption apparatus, and the data underwent analysis through the BET technique.For XRD analysis, a Bruker D8 Advance diffractometer was employed, with operational parameters set within a 2° to 10° range, utilizing Cu K radiation at 30 mA and 40 kV.FTIR analysis was carried out by PerkinElmer spectrum Two spectrometer, employing a blend of KBr and TMIP-206.

XRD
XRD analysis was performed to assess the crystallinity of TMIP-206 as well as water stability over a one month of water exposure.The presence of well-defined peaks in Fig. 1 signifies that TMIP-206 and MIP-206 exhibit a high level of crystallinity.The XRD pattern of TMIP-206 closely mirrors that of the original MIP-206, displaying peaks at 3.3°, 5.3°, 5.9°, 7.3°, 8.2°, 9.1°, and 10°, confirming the successful synthesis of MIP-206 in this study 47 .This resemblance implies that the modification did not compromise the material's structural integrity or crystalline nature.Additionally, Fig. 1 illustrates that both MIP-206 and TMIP-206 demonstrated excellent water stability during one month of water contact as they maintained their crystalline structure almost intact.

TGA
The results of TGA analysis for MIP-206 and TMIP-206 are presented in Fig. 4. The weight reduction below 230 °C corresponds to the loss of water, trapped solvent, and the release of formates.However, there is an extra weight reduction step in TMIP-206 below 230 °C corresponding to the decomposition of triethylenetetramine moieties.The thermal response of MOFs to temperature increase starts at approximately 230 °C.The structural decomposition of MIPs corresponded to organic portion stops at 525 °C, at which the remaining weight is attributed to inorganic ZrO2 portion.

BET
Figure 5-a shows the nitrogen adsorption-desorption isotherm of TMIP-206.According to IUPAC classification, the nitrogen adsorption isotherm exhibits a type IV shape, indicating the presence of mesopores within its structure.As per the BET analysis, the determined surface area for TMIP-206 was 1015 m 2 /g (Fig. 5a).This notable specific surface area significantly facilitates the rapid adsorption of lead ions on the formulated adsorbent.In addition, the total pore volume of the TMIP-206 was found to be 0.41 cm 3 g −1 (Fig. 5b).The pose size distribution of TMIP-206 indicates the presence of mesopores in the range of 2.1-3.3 nm, confirming the type IV isotherm type.The average pore size was observed to be 2.6 nm and Fig. 5-b shows a uniform size distribution.www.nature.com/scientificreports/

Lead adsorption isotherms and kinetics
The current investigation aimed to elucidate the adsorption capabilities of TMIP-206, for lead removal.Langmuir, Freundlich, and Temkin isotherm equations were employed to analyze the kinetic data.The kinetic data was subjected to analysis by Elovich, pseudo-first, and pseudo-second-order models.The Langmuir isotherm determined the maximum theoretical removal capacity of Pb as 250 mg/g (Fig. 6 and Table 1).However, experimental results indicated actual Pb uptake of 267.15 mg/g, revealing a higher real lead sorption performance compared to the theoretical capacity. Figure 7 and Table 2 revealed that the lead adsorption behavior adhered to the pseudo-2nd-order model, suggesting chemisorption in the removal process.The Langmuir model demonstrated good agreement with the adsorption isotherms on TMIP-206, indicating uniform surface adsorption characterized by a restricted quantity of identical sorption sites.Findings suggest the significant potential of the synthesized MOFs as useful adsorbents for lead adsorption from wastewater.Table 3 presents the effectiveness of comparable adsorbents for Pb adsorption, showcasing the superior adsorption capacities of TMIP-206 compared to other reported values.These synthesized adsorbents exhibit remarkable adsorption capacities for lead elimination.Moreover, the intra-particle diffusion model was utilized to examine the kinetic data (Fig. 8).According to the intra-particle kinetic model, a linear graph intersecting the origin indicates that intra-particle diffusion is the  only determining factor.Conversely, deviation from the origin on the graph implies the presence of additional processes influencing the rate, signifying that intra-particle diffusion alone does not govern the rate 51,52 .

Adsorption mechanisms
The FTIR spectra following the adsorption of metal ions are depicted in Fig. 3b to investigate the mechanism underlying Pb ion removal by TMIP-206 and the associated functional groups.Our examination showed a slight decrease in the peaks at 660, 1046, and 3301 cm −1 , pertaining to the bending vibration of the amine group, suggesting interaction between the amine group and lead ions.Additionally, a peak around 823 cm −1 , potentially indicating the formation of N-Pb bonds, emerged post-lead ion removal, indicative of lead-amine coordination bonds formation.In summary, the process of lead ion removal by TMIP-206 entails complex interactions between metal ions and amine functional groups.

PH study on lead adsorption
Prior research has demonstrated a significant influence of the initial solution pH on removal efficiency.Optimal adjustment of the pH not only minimizes matrix interference but also enhances the adsorption capacity to its maximum extent.The impact of pH was examined within a pH range of 2-8 at a lead concentration of 50 mg/L, and the outcomes are presented in Fig. 9.As indicated by the figure, the most effective pH for lead ion removal was determined to be 6.At lower pH values, an elevated concentration of hydrogen ions impedes the occupation of active adsorption sites by lead ions, resulting in a decrease in lead removal below pH 6. Conversely, with an increase in solution pH, the gradual creation of PbOH 2 hinders the adsorption of Pb ion on the adsorbent, making it challenging to monitor their presence in that particular region.

Adsorbent reusability
The regeneration of the adsorbent is a crucial aspect in in advancing a cost-effective adsorption process.The efficiency of Pb removal by the synthesized MOFs exhibited a slight decline with each successive of recycling sessions (Fig. 10).This fact can be ascribed to the gradual loss of pore volume during the successive adsorption-desorption cycles, wherein certain Pb particles may become entrapped within the adsorbent structure, impeding their complete removal even after regeneration.Despite this, the performance of the as-synthesized MOFs in Pb removal remained relatively stable, experiencing a reduction of less than 15% over the course of five cycles.The sustained effectiveness of the MOFs through multiple cycles of Pb adsorption and desorption underscores their durability and potential for long-term use.www.nature.com/scientificreports/www.nature.com/scientificreports/

Study of the impact of interfering ions on Pb uptake using TMIP-206
The utilization of TMIP-206 for the treatment of a real wastewater sample was performed to examine its applicability for real-life scenarios.Table 4 presents the physicochemical characteristics of an actual wastewater sample rich in Pb 2+ before and after the treatment process.TMIP-206 exhibited a high affinity for Pb 2+ ions and reduced its content from 56.6 to 0.3 mg/L, indicating its excellent performance for the removal of Pb 2+ ions.Additionally, some positively charged ions such as Cu 2+ and Fe 2+ were adsorbed on the surface of TMIP-206 due to similarity in charge and size.However, anions such as N O − 3 , P O 3− 4 , and SO 4 2− underwent negligible changes during the treatment due to repulsive forces from the negatively charged surface of TMIP-206.

Conclusion
In summary, a successful adsorbent was synthesized for the removal of Pb ions from water, utilizing TMIP-206.The isotherm analysis of lead adsorption revealed that the Langmuir model provides the most accurate description of the adsorption behavior.Notably, the Langmuir model yielded a maximum removal capacity of 267.15    7.6 mg L −1 6.5 mg L −1 Cl − 6.3 mg L −1 5.9 mg L −1

Figure 2 Figure 1 .
Figure 2 illustrates the SEM images of MIP-206 and TMIP-206.According to this figure, both MIPs exhibit crystals in irregular shapes with interconnected morphologies.The crystals have smooth surfaces with smaller crystal particles formed on the surface.Their crystal structure contributes to their stability and their interconnect morphologies add to their better mass transfer performance necessary for adsorption purposes.The size of the particles varies; however, most of the particles are in the range of 4-7 µm.After one month of water exposure

Figure 2 .
Figure 2. SEM images of MIP-206 (a) at day 0 of water exposure, (c) at day 30 of water exposure, TMIP-206 (a) at day 0 of water exposure, and (b) at day 30 of water exposure.

Figure 9 .
Figure 9. pH effect of TMIP-206 for lead ions adsorption process at C = 50 mg/L.

Table 3 .
Comparison of Pb removal capacity of recently investigated MOFs.

Table 4 .
Physiochemical properties of real water sample before and after adsorption using TMIP-206.